The Effect of Subjecting Peas to Air Enriched with Carbon Dioxide: II. RESPIRATION AND THE METABOLISM OF THE MAJOR ACIDS

Whole peas at about 75 per cent of their maximum fresh weightwere subjected to 5–30 per cent CO₂ in air for periodsof from 1–6 d, then returned to air for a further 1 d.Samples were withdrawn at intervals and organic acids, ^TCO₂and ethanol estimated as well as the rate of respiration. Slicesof cotyledons suspended in water were also subjected to highconcentrations of CO₂ in air for 3 h. The rate of respiration was inhibited progressively by increasein CO₂ content of the tissue. The high internal CO₂ contentof the intact pea causes an inhibition of its rate of respirationby about 25 per cent. Alcohol production commenced at between10 and 15 per cent CO₂ in the ambient gas and slowly increasedin rate up to 37 per cent. The CO₂-air mixtures reduced the content of malate, pyruvateand -oxoglutarate, increased that of succinate and left citrateunaffected. On return to air malate rose rapidly and succinatefell slowly to their original concentrations. During the sameperiod the concentration of PEP fell sharply and after about1 h rose again, whereas oxalacetate showed a reverse response.It is argued that the rapid re-synthesis of malate was by carboxylationof PEP to oxalacetate and that this reaction was stimulatedby a change in pH rather than by the direct effect of the changein concentration of CO₂. In one experiment ¹⁴CO₂ was supplied for 2 h before return toair and the movement of ¹⁴C followed for 6 h. The results supportthe method of re-synthesis of malate proposed.     

Carbon Dioxide Exchange in Lichens: Apparent Photorespiration and Possible Role of CO2 Refixation in Some Members of the Stictaceae (Lichenes)

At low O₂ concentration (1% O₂, v/v) the true photosynthetic(TP) rates of Pseudocyphellaria homoeophylla and Sticta latifronsare stimulated by up to 46% and 55% respectively. This stimulation,which is considered to be due to the repression of photorespiration(PR), varied with the thallus water content of the lichens.At intermediate and low water contents PR rates approximatedthe 43% of TP theoretically expected for C3 plants, but at veryhigh water contents PR rates were near zero. Results for P.billardierii show that in this species PR rates are low (upto 11% TP) at all water contents. Comparison of light/dark CO₂ evolution ratios under low O₂ conditionsindicated that up to 70% of respired CO₂ could be refixed withinthe lichen thallus. The lack of stimulation at 1% O₂ and highwater content is suggested to be a result of a high surfaceCO₂ diffusion resistance encouraging internal CO₂ refixation.Lichens thus join the group of plants with low CO₂ compensationvalues and photorespiration.     

A Comparison of the Rate of Maintenance Respiration in Some Crop Legumes and Tobacco Determined by Three Methods

Carbon exchange was measured on whole plants of field bean,lucerne, chick pea, kidney bean, pea and tobacco. The maintenance respiration rate was measured in three ways:(i) by allowing the CO₂ efflux to decay in prolonged darknessto an asymptotic value which was then taken to be the maintenancevalue (the dark decay method); (ii) by plotting the dark CO₂efflux as a function of the net CO₂ uptake over a range of irradiancesand taking maintenance as the dark CO₂ efflux when the net CO₂uptake was zero (the dynamic method); and (iii) by plottingthe total CO₂ uptake as a function of the growth rate and takingmaintenance respiration as the CO₂ efflux when the growth ratewas zero (the zero growth rate method). The range of valuesfor the maintenance coefficient over all species was from 1.6to 2.1 per cent of the dry weight per day, 1.8 to 2.1 per centand 2.7 to 2.9 per cent as determined by these three methodsrespectively. There was a linear relationship, common to allspecies, between the maintenance respiration rate (dark decaymethod) and dry weight, total nitrogen and the organic nitrogencontent. The growth coefficient (0.69±0.01) was the samefor field bean, chick pea and lucerne and was unaffected bythe method of estimation. It was concluded that the dark decay method provided the bestestimate of the minimal maintenance requirements in the plantsstudied. Vicia faba L., Medicago sativa L., Cicer arientinum L., Phaseolus vulgaris L., Pisum sativum L., Nicotiana tobacum L., field bean, lucerne, chick pea, kidney bean, pea, tobacco, respiration, maintenance, growth, nitrogen content     

Calcification in the Green Alga Halimeda: III. THE SOURCES OF INORGANIC CARBON FOR PHOTOSYNTHESIS AND CALCIFICATION AND A MODEL OF THE MECHANISM OF CALCIFICATION

Calcification and photosynthetic rates in Halimeda tuna weremeasured by the ¹⁴C method under conditions of differing pHand total inorganic carbon (CO₂) concentrations. The effectsof pH and CO₂ on photosynthesis and respiration were also monitoredwith a polarographic O₂ electrode. The results obtained indicatethat the intercellular pH and CO₂ differ from those of the externalmedium. Experiments carried out over a range of pH values show thatHalimeda can use for photosynthesis. Photosynthesis appears to stimulate calcification by removing CO₂ from theintercellular spaces. As these spaces are isolated from theexternal sea water by the layer of cell wall of the adpressedperipheral utricles, the removal of CO₂ results in a rise in[] and a rise in pH. This results in an increased rate of CaCO₃ precipitation. Respiratory CO₂ evolution has aninhibitory effect on calcification by decreasing the pH and[]. A model for calcification in Halimeda is proposed based on theresults of this and previous papers. Calcification in Halimedais seen to be a result of the anatomy of the thallus in whichthe sites of calcification are within a semi-isolated chamberwhere removal or addition of CO₂ due to photosynthesis or respirationcan effectively change [CO] thereby resulting in precipitation of CaCO₃. In the Appendix to this paper theoreticalcalculations illustrate the effects of CO₂, , and removal or addition in a closed system on the relative concentrations of the other inorganic carbonspecies.     

A Method for Separating Cuticular and Stomatal Components of Gas Exchange by Amphistomatous Leaves: I MODEL SOLUTION

A new technique for estimating the cuticular component of epidermalgas exchange by a stomatous leaf side is proposed. It is basedon the process of elimination of stomatal diffusion by mass(viscous) flow of air applying an air pressure gradient acrossthe leaf. This technique was designed to enable a reliable estimationof the cuticular component irrespective of stomatal opening. A model solution of diffusive and mass flow counteraction interms of general substance fluxes is presented. Water vapourloss and CO₂ uptake by a model leaf was simulated by varyingboth stomatal diffusion resistance and viscous flow of air throughthe stomatal pores in physiologically and experimentally relevantranges. Depending on stomatal opening, elimination of the stomatalcontribution to epidermal vapour and CO₂ exchange by the viscousflow of air ranged from small to practically complete. It supportsthe relevance of the procedure for cuticular vapour loss estimationunder conditions of partially open stomata. Modification of the model CO₂-uptake patterns due to expectedchanges in intercellular CO₂ concentration, , was evaluated. Net CO₂ flux under the diffusive-viscous flowscounteraction is sensitive to the changes mentioned above. Nevertheless,the changes in , evaluated by a simple model, were too small to cause significant departures from the CO₂-uptakeelimination curves by constant . The relevance of the method for the determination of cuticular CO₂-uptakeis discussed. Key words: Cuticular transpiration, cuticular CO₂-uptake, methods, diffusive-viscous flows counteraction, model     

Carbon Metabolism in Seagrasses: I. THE UTILIZATION OF EXOGENOUS INORGANIC CARBON SPECIES IN PHOTOSYNTHESIS

Four species of seagrasses, Halophila stipulacea, Thalassodendronciliatum, Halodule uninervis, and Syringodium isoetifolium,were investigated for their ability to utilize and CO₂ as exogenous carbon sources for photosynthesis. Ratesof photosynthesis were measured as rates of O₂ evolution ina closed system in which the pH was continuously controlled.A computer program was written to calculate the concentrationsof different carbon species as a function of pH and other specifiedexperimental conditions. Bicarbonate as well as CO₂ were readily assimilated by all fourseagrass species. Saturating concentrations of , at saturating light intensities, were 0.5–1.8 mM dependingon the species. Rates of photosynthesis under such conditionswere 0.1–0.55 µmol O₂ min^–1 mg^–1 chlorophyll.At saturating CO₂ concentrations, i.e. 0.5–1.3 mM, ratesof photosynthesis were 0.22–1.4 µmol CO₂ min^–1mg^–1 chlorophyll. Photosynthetic rates in each specieswere considerably higher when CO₂ rather than was supplied at saturating concentrations. The concentration of in natural seawater was found to be saturating, and that of CO₂ insufficient forconsiderable photosynthetic rates in these plants under thegiven conditions It was thus concluded that is the major carbon source for photosynthesis in seagrasses.     

STUDIES ON THE RE-ASSIMILATION OF RESPIRATORY CO2 IN ILLUMINATED LEAVES

Experiments were performed, using rice, barley and Hydrangealeaves, to examine the re-assimilation of respiratory ¹⁴CO₂while photosynthesis is going on in an open air flow system. It was found that the leaves which had assimilated ¹⁴CO₂ beforehandevolved, when kept under photosynthesizing conditions, threeto four tenths (variable according to plant species and externalconditions) of the amount of ¹⁴CO₂ to be produced in the dark.Such an incomplete re-utilization of ¹⁴CO₂ was observed alsoin spinach leaf homogenate as well as in the leaves which hadpreviously absorbed ¹⁴C-glucose. The ¹⁴CO₂ output in rice leaves was found to be acceleratedby the light of high intensity. A possibility of light stimulationon the respiration was suggested. (Received October 7, 1961; )     

Measurement of the Carbon Dioxide Compensation Point and the Rate of Loss of 14CO2 in the Light and Dark in Some Bryophytes

The CO₂ compensation point at 25 °C and 250 µEinsteinsm^–2 s^–1 wasmeasured for 27 bryo-phyte species, andwas found to be in the range of 45–160 µl CO₂ I^–1air. Under the same conditions Zea mays gave a value of 11 µlI^–1 and Horde um vulgare 76 µI^–1. The rate of loss of photosyntheticallyfixed ¹⁴CO₂ in the light and dark in six bryophytes (three mosses,two leafy liverworts, one thalloid liverwort) was determinedin CO₂-free air and 100% O₂. The rate of ¹⁴CO₂ evolution inthe light was less than that in the dark in CL₂-free air, butin 100% O₂ the rate in the light increased, so that in all butthe leafy liverworts it was greater than that in the dark. Raisingthe temperature tended to increase the rate of 14CO₂ evolutioninto CO₂-free air both in the light and dark, so that the light/dark(L/D) ratio did not greatly vary. The lower rate of loss of¹⁴CO₂ in the light compared tothe dark could be due to partialinhibition of ‘dark respiration’ reactions in thelight, a low rate of glycolate synthesis and oxidation, or partialreassimilation of the ¹⁴CO₂ produced, or a combination of someor all of these factors.     

STUDIES ON THE METABOLISM OF A SULFUR-OXIDIZING BACTERIUM III. POSTOXIDATIVE FIXATION OF CARBON DIOXIDE

1. The cells of Thiobacillus thiooxidans, which had been in contactwith sulfur or sulfide in air (or CO₂-free air), could fix addedCOa very rapidly after replacing air with nitrogen. This fixationis designated as the postoxidative fixation.
2. "Preoxidation"of the sulfur compounds is mandatory for theoccurrence of thepostoxidative fixation.
3. The cells which had preliminarilyoxidized sulfide could notshow the CO₂-fixation, when theywere placed under an anaerobiccondition in the absence of thesulfur compound.
4. These results indicate that sulfur compoundsmay have an importantrole as the electron donor for the reductionof CO₂, besidestheir role as the substrate of respiration tosecure energyfor the fixation of CO₂

(Received March 6, 1962; )     

Dormancy in Cereal Seeds: II. THE NATURE OF THE GASEOUS EXCHANGE IN IMBIBED BARLEY AND RICE SEEDS

When barley seeds imbibe water, the O₂ uptake of non-dormantseeds is considerably less than that of dormant seeds for atleast the first 6 h, irrespective of the rate at which the seedshad previously lost dormancy. During the initial 6 h of imbibition, the CO₂ output of dormantbarley seeds is usually only slightly greater than and sometimesno different from that of nondormant seeds. The CO₂ output ofdormant seeds is reduced by about 66 percent by millimolar KCN,whereas that of non-dormant seeds is decreased by about 12–13per cent only. The CO₂ output of dormant barley in nitrogenis considerably less than the CO₂ output of non-dormant seedsunder the same conditions. Dormant rice seeds also show a higher initial O₂ uptake thannon-dormant seeds, though this is not generally as marked asin barley. Similarly, the initial CO₂ output of dormant seedsis distinctly greater than that of non-dormant seeds, but inmillimolar KCN it is depressed to a greater extent than in non-dormantseeds. In nitrogen, the CO₂ outputs of dormant and non-dormantseeds were found to be the same. Consequently, unlike barley,dormant rice seeds appear to be as capable of carrying out alcoholicfermentation under anaerobic conditions as nondormant seeds. In barley, increasing the O₂ tension from 21 per cent to 100per cent increased the oxygen uptake of dormant seeds more thanthat of non-dormant seeds (an increase of 53 per cent as against20–23 Per cent). In dormant seeds there was a concomitantincrease in CO₂ output (about 50 per cent), but the CO₂ outputof non-dormant seeds was hardly affected. High concentrations of CO₂ are inhibitory to the germinationof both dormant and non-dormant barley seeds. At a concentrationof 10 per cent, however, CO₂ is inhibitory only to dormant seeds,although at 2.5–5 per cent it is sometimes stimulatoryto the germination of dormant seeds. A 24–h treatmentwith appropriate concentrations of ethanol, lactic acid, oracetaldehyde is also stimulatory to the germination of dormantbarley seeds. Histochemical investigations in barley indicated the presenceof peroxidase, cytochrome oxidase, and -glycero-phosphate dehydrogenasein the embryo, aleurone layer, and in a layer associated withthe testa. A number of other redox enzymes were detected inthe embryo and aleurone layer only. No differences in distributionor intensity of activity were detected between dormant and nondormantseeds.     

线性和指数回归方法对土壤呼吸CO2扩散速率估算的影响

 理论上,土壤呼吸通量的量值可以通过观测土壤呼吸CO₂扩散速率(əc/ət)计算得到。但是为获得əc/ət,通常须允许土壤呼吸箱内CO₂浓度升高,因此,如何估算外界大气CO₂浓度条件下的əc/ət是土壤呼吸观测技术的关键,关系到观测结果的准确性。通常əc/ət的估算会受土壤表层大气CO₂扩散梯度(即土壤呼吸箱内CO₂扩散梯度和大气CO₂浓度昼夜变化)的影响。目前,线性回归方法是土壤呼吸观测中估算əc/ət的基本方法。然而,常用的线性回 归方法会低估əc/ət,而指数回归方法则可以准确地估算əc/ət。夜间əc/ət的变化与大气CO₂ 浓度之间存在非常明显的负相关关系。夜间土壤表层大气CO₂扩散梯度的减小导致线性回归方法明显低估əc/ət。əc/ət的昼夜变化过程存在明显的非对称性现象,而指数回归方法可以更好地描述əc/ət昼夜变化的非对称性响应。     

Studies in the Respiratory and Carbohydrate Metabolism of Plant Tissues1: XVII. THE EFFECT OF IODOACETATE ON THE CO2 OUTPUT, PATHWAYS OF GLUCOSE RESPIRATION AND CONTENTS OF PHOSPHATE ESTERS IN STRAWBERRY LEAVES

When specifically labelled glucose was fed to strawberry leaves,the C₆/C₁, quotient (rate of release of ¹⁴CO₂ from glucose-6-¹⁴C/rateof release of ¹⁴CO₂ from glucose-1¹⁴C ranged from 0.27 to 0.35in leaves in water and from 0.46 to 0.96 in leaves fed withiodoacetate. These quotients indicate that both the glycolyticand the pentose phosphate pathways participate in the respirationof strawberry leaves, with a greater contribution from the formerin the iodoacetate increased CO₂ output. Concurrently with the increase of CO² output in iodoacetate,the contents of glucose-6-phosphate (G6P), fructose-6 (F6P)and fructose-1,6-diphosphate (FDP) increased greatly; therewas a smaller increase of phosphoenol-pyruvate (PEP). The increasein the CO₂ output in iodoacetate may be explained solely onthe basis that the increases of G6P and FDP accelerate the ratesrespectively of the pentose phosphate pathway and of glycolysisand traffic into the tricarboxylic acid cycle. The increasein content of G6P and FDP is attributed to an increase in theaccessibility of enzymes and substrates caused by iodoacetate.Alternatively the increased CO₂ output in iodoacetate may bepartly due to uncoupling of oxidative phosphorylation.     

The Labelling of Glucose-6-phosphate in Pea Slices Supplied with 14C Glucose, and the Assessment of the Respiratory Pathways in Air, in 10% CO2 and in 2% O2 in N

A method is proposed for estimating the proportion of CO₂ ofrespiration derived from the PPP and the TCAC by measuring thespecific activity of the respired CO₂ and those of its precursorsin the tissue. In practice the specific activity of C-1 of G-6-Pand of the carboxyl carbon atoms of pyruvate and malate havebeen taken as the precursors. To apply this method there mustbe good evidence that these compounds are not in ‘pools’in the cell. In air in the sliced pea cotyledon 25–40% of the respiredCO₂ arose from the PPP. In both 10% CO₂ in air and in 2% O₂ in N there was a showingin the rate of respiration and a large increase in the proportionof the CO₂ deriving from the PPP. It is argued that in eachcase the cause of this was different. Key words: Carbon dioxide, Respiration, Pisum Sativum, Pentose-phosphate pathway     

Effect of Temperature on Photosynthesis and Photorespiration of Wheat Leaves

Wheat plants were grown in a controlled environment with daytemperatures of 18 ?C and with 500 µ Einsteins m^–28^–1 of photosynthetically active radiation for 16 h. Beforeanthesis and 2 to 3 weeks after, rates of net photosynthesiswere measured for leaves in 2 or 21% O₂ containing 350 vpm CO₂at 13, 18, 23, and 28 ?C and with 500 µEinsteins m^–2s^–1 of photosynthetically active radiation. Also, underthe same conditions of light intensity and temperature, therates of efflux of CO₂ into CO₂-free air were measured and,for mature flag leaves 3 to 4 weeks after anthesis, gross andnet photosynthesis from air containing 320 vpm ¹⁴CO₂ of specificactivity 39?7 nCi µmol^–1. When the O₂ concentration was decreased from 21 to 2% (v/v)the rate of net photosynthesis increased by 32 per cent at thelowest temperature and 54 per cent at the highest temperature.Efflux of CO₂ into CO₂-free air ranged from 38 per cent of netphotosynthesis at 13 ?C to 86 per cent at 28 ?C. Gross photosynthesis,measured by the ¹⁴C assimilated during 40 s, was greater thannet photosynthesis by some 10 per cent at 13 ?C and 17 per centat 28 ?C. These data indicate that photorespiration was relativelygreater at higher temperatures.     

准噶尔荒漠早春短命植物的光合特性及生物量分配特点

准噶尔荒漠分布的早春短命植物不仅具有十分独特的生物学特点,而且在荒漠植物群落演替、物种多样性维持及土壤改良与防治水土流失等方面具有重要的生态学价值。该文运用Li-6400开放式气体交换光合作用测定系统,对分布于准噶尔荒漠的16种早春短命植物生长盛期的净光合速率(P_n)、蒸腾速率(T_r)、水分利用效率(WUE)等特征进行了测定,并对其中7种植物与生长相关的生物量分配特征进行了分析。结果表明:1)16种植物的最大P_n、 最大T_r及WUE分别为8.07~35.96 μmol CO₂·m^-2·s^-1、3.16~29.64 mmol H₂O·m^-2·s^-1、0.54~4.26 μmol CO₂·mmol^-1H₂O;种间最大P_n与最大气孔导度(Stomatal conductance, G_s)之间存在正相关关系,其相关系数为0.77(p<0.05),线性回归斜率为26.36 μmol·mmol^-1;从光合速率对胞间CO₂浓度及光量子通量密度的响应曲线来看,这类植物的表观CO₂补偿点均在4~5 Pa之间(28~30 ℃),表观羧化效率为0.64~1.86 μmol CO₂·m^-2·s^-1·Pa^-1,表观量子效率为0.05~0.06。2)从生物量分配来看,所测植物的个体生物量为0.05~0.39 g;单株总叶面积为 3.24~51.40 cm²;单位叶面积干重为0.40~0.77 g·m^-2,根在总生物量中所占比例为5.72%~19.43%,单株叶面积比在2.92~9.00 m²·kg^-1之间。种间根所占生物量的比与对应的WUE之间的比较分析结果表明,二者之间存在显著的正相关关系,其相关系数r为0.93(p<0.01)。这些结果表明,所观测的早春短命植物具有典型的C₃植物特征,相比其它类型的荒漠植物具有较高的单位叶面积P_n、高T_r及低WUE,并且在生长发育过程中表现出很低的根/地上生物量比、较高的叶面积比和单位叶面积干重,说明它们具有相对高的生长速率,这与其生长发育节律相一致,反映了它们与准噶尔荒漠环境相适应的特点。     

Active uptake of inorganic carbon by Chlorella saccharophila is not repressed by growth in high CO2

Regulation of transport of dissolved inorganic carbon (DIC)in response to CO₂ concentration in the external medium hasbeen compared in two closely-related green algae, Chlorellaellipsoidea and Chlorella saccharophila. C. ellipsoidea, whengrown in high CO₂, had reduced activities of both CO₂ and transport and DIC transport activitieswere increased after the cells had acclimated to air. However,high CO₂-grown C. saccharophila had a comparable level of photosyntheticaffinity for DIC to that of air-grown C. ellipsoidea and thiswas accompanied by a capacity to accumulate high internal concentrationsof DIC. The high photosynthetic affinity and the high intracellularDIC accumulation did not change in cells grown in air exceptthat the occurrence of external carbonic anhydrase (CA) in air-grownC. saccharophila stimulated the intracellular DIC accumulationin the absence of added CA. These data indicate that activeDIC transport is constitutively expressed in C. saccharophila,presumably because this alga is insensitive to the repressiveeffect of high CO₂ on DIC transport. This strongly supportsthe existence of a direct sensing mechanism for external CO₂in Chlorella species, but also indicates that external CA isregulated independently of DIC transport in Chlorella species. Key words: Carbonic anhydrase, Chlorella, CO₂-insensitive, DIC transport, wild type     

Studies of the Mechanism by which Daminozide (B9) Inhibits Stomatal Opening

The mechanism by which the synthetic growth retardant daminozide(B9) inhibits stomatal opening has been investigated, and itsaction compared with that of the naturally occurring growthhormone, abscisic acid. The inhibition of stomatal opening bylow concentrations of daminozide in normal air is largely reversedin CO₂-froe air, suggesting that the main effect is due to anincrease in internal CO₂ concentratiom This is supported byobservations of a raised level of CO₂ compensation in treatedleaves. The CO₂-independent light reaction of stomata does,however,also appear to be inhibited by dammozide. Higher concentrationscause damage to cells of the epidermis, and such effects couldalso be found in normal practical use of the growth retardant. The mechanism of action of daminozide on stomata has littlein common with that of abscisic acid, which does not operateby increasing the internal CO₂ concentration and does not inducecellular damage. Daminozide cannot be recommended for practicaluse as an antitrans-pirant because of its mechanism of action,which is likely to cause a decrease in the efficiency of wateruse.     

Delay in the Response of Stomata to Abscisic Acid in CO2-free Air

Abscisic acid (10^–5 M) was fed via their petioles to leavesdetached from well watered plants of Xanthium strumartum, whilethe intercellular spaces were flushed with air of known CO₂content. A closing response to ABA occurred in the presenceor absence of CO₂, and the stomata responded to CO₂ whetheror not ABA was supplied to the leaves. A factorial experimentrevealed no interaction between CO₂ and ABA, and suggested thattheir effect on the rate of closure was purely additive. Theonly evidence of interdependence between the two corn poundswas a delay in the response to ABA in C0 air, which was moremarked in a high light intensity. A hypothesis which is consistentwith the data is that ABA induces stomatal closure by interferingwith the energy supply required for the active transport processeson which guard cell turgor depends. The inhibitory action ofABA takes longer in CO₂-free air because, in the absence ofCO₂ fixation, energy is available from chioroplasts as wellas mitochondria.     

Sources of Inorganic Carbon Acquired through CAM in Littorella uniflora (L.) Aschers

Madsen, T. V. 1987. Sources of inorganic carbon acquired throughCAM in Littorella uniflora (L.) Aschers.—J. exp. Bot.38: 367–377. The CO₂ dynamics of the lacunal air and the relative contributionof external and internal CO₂ sources to dark CO₂ assimilationwas examined in the submerged aquatic CAM species Littorellauniflora (L.) Aschers. Refixation of internal CO₂, released by dark respiration, constitutedabout 30–35% of the total dark CO₂ assimilation. At aCO₂ concentration of 0·2 mol m^–3 around the leavesthe external CO₂ uptake through the roots increased from 45%of the total CO₂ uptake at 0·7 mol m^–3 CO₂ to 100%at 1·6 mol m^–3 and 3·1 mol m^–3 CO₂around the roots. The negligible importance of leaf CO₂ uptakeat high CO₂ concentrations around the roots was the result ofa causative high CO₂ concentration in the leaf lacunae. The CO₂ permeability of Littorella leaves was high relativeto root permeability. This has at least two ecological implications:(1) it enhances the potential diffusive release of CO₂ fromthe sediment C0₂-pool via the lacunal system of the plants.This loss of CO₂, however, was found to be greatly reduced byCAM activity of the plants. (2) The high permeability of theleaf surface to CO₂ exchange allows the plants to assimilateCO₂ from the water surrounding the leaves when the concentrationis high, i.e. during extensive epiphyte dark respiration. Thus,CAM tends to facilitate retension of a high CO₂ pool in thesediment-plant system and at the same time allows the plantsto exploit the water column CO₂ source when it is abundant.This result is in accordance with the general idea that CAMin aquatics constitute a carbon conserving mechanism. Key words: Aquatic macrophytes, dark CO₂ assimilation, inorganic carbon sources     

Effect of carbon dioxide concentration on pigmentation in the blue-green alga Anacystis nidulans

The pigment content in the blue-green alga Anacystis nidulanswas found to be dependent upon CO₂ concentration during growth.In cells grown with 1% CO₂ in air the total pigment constituted20.5% of the dry weight while it was only 11.1% of dry weightof cells grown in air (0.03% CO₂). This decrease in total pigmentwas found to be almost entirely ascribable to decrease in phycocyanin.Since light absorbed by phycocyanin has been shown to providenearly equal rates of photoreactions I and II, the "CO₂ control"of phycocyanin is viewed as an effective means of regulationof the photoreactions without upsetting the balance of operationof the two photoreactions. (Received December 25, 1970; )